The present invention relates to limited slip differentials, and more particularly to limited slip differentials having an electromagnetically actuated clutch.
Differentials are well known in the prior art and allow each of a pair of output shafts or axles operatively coupled to a rotating input shaft to rotate at different speeds, thereby allowing the wheel associated with each output shaft to maintain traction with the road while the vehicle is turning. Such a device essentially distributes the torque provided by the input shaft between the output shafts.
The completely open differential, i.e., a differential without clutches or springs which restrict relative rotation between the axles and the rotating differential casing, is not well suited to slippery conditions in which one driven wheel experiences a much lower coefficient of friction than the other driven wheel: for instance, when one wheel of a vehicle is located on a patch of ice and the other wheel is on dry pavement. Under such conditions, the wheel experiencing the lower coefficient of friction loses traction and a small amount of torque to that wheel will cause a xe2x80x9cspin outxe2x80x9d of that wheel. Since the maximum amount of torque which can be developed on the wheel with traction is equal to torque on the wheel without traction, i.e. the slipping wheel, the engine is unable to develop any torque and the wheel with traction is unable to rotate. A number of methods have been developed to limit wheel slippage under such conditions.
Prior means for limiting slippage between the axles and the differential casing use a frictional clutch mechanism, either clutch plates or a frustoconical engagement structure, operatively located between the rotating case and the axles. Certain embodiments of such prior means provide a clutch element attached to each of the side gears, and which frictionally engages a mating clutch element attached to the rotating casing or, if the clutch is of the conical variety, a complementary interior surface of the casing itself. Such embodiments may also include a bias mechanism, usually a spring, to apply an initial preload between the clutch and the differential casing. By using a frictional clutch with an initial preload, a minimum amount of torque can always be applied to a wheel having traction, e.g., a wheel located on dry pavement. The initial torque generates gear separating forces between the first pinion gears and the side gears intermeshed therewith. The gear separating forces urge the two side gears outward, away from each other, causing the clutch to lightly engage and develop additional torque at the driven wheels. Examples of such limited slip differentials which comprise cone clutches are disclosed in U.S. Pat. No. 4,612,825 (Engle), U.S. Pat. No. 5,226,861 (Engle) and U.S. Pat. No. 5,556,344 (Fox), each of which is assigned to Auburn Gear, Inc., the disclosures of which are all expressly incorporated herein by reference.
Certain prior art limited slip differentials provide, between the first of the two side gears and its associated clutch element, interacting camming portions having ramp surfaces or ball/ramp arrangements. In response to an initiating force, this clutch element is moved towards and into contact with the surface against which it frictionally engages, which may be a mating clutch element attached to the casing, or an interior surface of the casing itself, as the case may be, thereby axially separating the clutch element and its adjacent first side gear, the interacting camming portions slidably engaging, the rotational speed of the clutch element beginning to match that of the differential casing due to the frictional engagement. Relative rotational movement between the ramp surfaces induces further axial separation of the clutch element and the first side gear. Because the clutch element is already in abutting contact with the surface against which it frictionally engages, the first side gear is forced axially away from the clutch element by the camming portions.
Certain embodiments of such limited slip differentials utilize an electromagnet having an electrical coil to effect the initiating force and actuate the clutch, as disclosed in U.S. Pat. No. 5,989,147 (Forrest et al.), U.S. Pat. No. 6,019,694 (Forrest et al.), and U.S. Pat. No. 6,165,095 (Till et al.), each of which is assigned to Auburn Gear, Inc., the disclosures of which are all expressly incorporated herein by reference. Each of these references discloses that the differential casing, in which the clutches are disposed, rotates within the housing and is rotatably supported by a pair of bearings. An electromagnet, which actuates a primary cone clutch element, is mounted in fixed relationship to the axle housing and is rotatably supported by the differential casing. Alternatively, as disclosed in pending U.S. patent application Ser. No. 09/484,967, filed Jan. 18, 2000, which is assigned to Auburn Gear, Inc., the disclosure of which is expressly incorporated herein by reference, the electromagnet may be fixedly supported by the axle housing. In either case, activation of the electromagnet draws a primary cone clutch element into frictional engagement with the rotating differential housing.
The camming portions, described above, act between the primary cone clutch element and the first side gear to axially separate them, forcing the first side gear into abutment with a transfer block located intermediate the first and second side gears. Responsive to this force, the transfer block is moved into abutment with the second side gear, which is rotatably fixed to a secondary cone clutch element, which frictionally engages a mating interior surface of the rotating differential casing. The frictional engagement of the secondary cone clutch element and the differential casing effects further clutched engagement between the axles and the differential casing, enhancing the locking capability of the limited slip differential. Notably, the load carrying capability of the secondary cone clutch mechanism is usually significantly greater than that of the primary cone clutch mechanism, owing to a greater axial engagement force exerted thereon. Examples of prior limited slip differentials are described in more detail below, with reference to FIGS. 1 and 2.
FIG. 1 depicts an embodiment of prior axle assembly 10 having electrically or electromagnetically actuated limited slip differential assembly 12. Axle assembly 10 may be a conventional axle assembly or comprise part of a transaxle assembly. Therefore, it is to be understood that the term xe2x80x9caxle assemblyxe2x80x9d encompasses both conventional (rear wheel drive) axle assemblies as well as transaxle assemblies. Differential assembly 12 comprises electromagnet 14, ferrous rotatable casing 16 constructed of joined first and second casing parts 16a and 16b, respectively, and providing inner cavity 18, which is defined by the interior surface of the circumferential wall portion of first casing part 16a and end wall portions 20, 22 of first and second casing parts 16a, 16b, respectively. Casing part 16a may be a machined iron or steel casting; casing part 16b may also be such a casting, or a ferrous, sintered powdered metal part. Disposed within cavity 18 are side gears 24, 26 and pinion gears 28, 30. The teeth of the side gears and pinion gears are intermeshed, as shown. Pinion gears 28, 30 are rotatably disposed upon cylindrical steel cross pin 32, which extends along axis 34. The ends of cross pin 32 are received in holes 36, 38 diametrically located in the circumferential wall of casing part 16a. 
Axles 40, 42 are received through hubs 44, 46, respectively formed in casing end wall portions 20, 22, along common axis of rotation 48, which intersects and is perpendicular to axis 34. Axles 40, 42 are respectively provided with splined portions 50, 52, which are received in splines 54, 56 of side gears 24, 26, thereby rotatably fixing the side gears to the axles. The axles are provided with circumferential grooves 58, 60 in which are disposed C-rings 62, 64, which prevent the axles from being removed axially from their associated side gears. The terminal ends of the axles 98 and 100 may abut against the cylindrical surface of cross pin 32, thereby restricting the axles"" movement toward each other along axis 48.
Primary clutch element 66 is attached to side gear 24 and rotates therewith. Clutch element 66 is ferrous and of the cone clutch variety and has frustoconical surface 68 which is adjacent to, and clutchedly interfaces with, complementary surface 70 provided on the interior of casing part 16a. Secondary clutch element 72 is also of the cone clutch variety and has frustoconical surface 74 which is adjacent to, and clutchedly interfaces with, complementary surface 76 also provided on the interior of casing part 16a. Cone clutches 66 and 72 may be of the type described in U.S. Pat. No. 6,076,644 (Forrest et al.) or U.S. Pat. No. 6,261,202, each of which is assigned to Auburn Gear, Inc., the disclosures of which are both expressly incorporated herein by reference, or may also be of any other suitable structure.
Disposed between primary cone clutch element 66 and side gear 24 is annular cam plate 78, which abuts thrust washer 82 adjacent end wall portion 22. Ball and ramp arrangement 84, 86, 88 is comprised of a first plurality of paired spiral slots 84, 86 located in cam plate 78 and primary cone clutch element 66, respectively. Slots 84, 86 define a helically ramping path followed by ball 88, which may be steel, disposed in each slot pair and a first ramp angle. With electromagnet 14 de-energized, balls 88 are seated in the deepest portion of slots 84, 86 by Belleville spring 90. The actuation sequence is created by the momentary difference in rotational speed between cone clutch element 66 and cam plate 78 as frustoconical surfaces 68 and 70 seat against each other. A more detailed discussion of ball/ramp camming arrangements is disclosed in U.S. Pat. No. 5,989,147.
In operation, a variable coil current on electromagnet 14 induces a variable amount of magnetic clamping force between casing part 16a and primary cone clutch element 66, which induces a variable amount of torque to be exerted by casing part 16a on clutch element 66. As electromagnet 14 is activated, axial separation of primary cone clutch element 66 and cam plate 78 is induced as cone clutch element 66 is magnetically pulled to the left against the force of Belleville spring 90 into clutched engagement with casing part 16a through frustoconical surfaces 68 and 70. In response to the initial flow of magnetic flux, cone clutch element 66 is pulled by the magnetic field to the left and surfaces 68 and 70 abut, and enter frictional engagement. As cone clutch element 66 and cam plate 78 separate axially, balls 88 are caused to rotate along the ramping helical paths of slots 84, 86 due to the relative rotation between clutch element 66 and cam plate 78. Cam plate 78 is urged against thrust washer 82 by the force of Belleville spring 90 and gear separation forces between pinion gears 28, 30 and side gear 24. As balls 88 rotate further along the helical ramp paths, frustoconical surfaces 68, 70 are forced into tighter frictional engagement and cam plate 78, still abutting thrust washer 82, reaches the end of its rotational travel relative to cone clutch member 66.
First side gear 24 moves towards the right, forcing secondary cone clutch element 72 into abutment with casing part 16a via transfer block 92 and second side gear 26 in the manner described above. Transfer block 92, which may be steel, is disposed about cross pin 32 and adapted to move laterally relative thereto along axis 48 to transfer movement of first side gear 24 to second side gear 26, thereby engaging secondary clutch element 72. As shown, transfer block 92 is attached directly to cross pin 32, and supports the cross pin in position within the differential casing as described in U.S. Pat. No. 6,254,505, assigned to Auburn Gear, Inc., the disclosure of which is expressly incorporated herein by reference. Alternatively, the transfer block may be loosely fitted about the cross pin, the cross pin being directly attached to the differential housing by a bolt extending through one end of the cross pin, as shown, for example, in U.S. Pat. No. 5,226,861. The shear loads associated with torque transmission are exerted on cross pin 32 near its opposite ends, particularly between the circumferential wall of casing part 16a and the adjacent pinion gears 28, 30.
Transfer block 92 includes opposite bearing sides 94, 96 which respectively abut first and second side gears 24, 26, and allows terminal ends 98, 100 of axles 40, 42, respectively, to abut the cylindrical side surface of cross pin 32. Transfer block 92 moves laterally relative to cross pin 32, along axis 48, such that rightward movement of side gear 24, described above, is transferred to side gear 26. Thus, during actuation of electromagnet 14, first side gear 24 is urged rightward, as viewed in FIG. 1, into abutting contact with transfer block 92. Transfer block 92 moves rightward, into abutting contact with second side gear 26; and second side gear 26 moves rightward, urging surface 74 of secondary clutch element 72 into frictional engagement with surface 76 of casing part 16a, thereby providing additional torque transfer capacity to the differential than would otherwise be provided with single cone clutch element 66.
Provided on the exterior surface of casing part 16a, near electromagnet 14, is flange 102, to which ring gear 104 is attached. The teeth 136 of ring gear 104 are in meshed engagement with the teeth of pinion gear 106 which is rotatably driven by an engine (not shown), thus rotating differential casing 16 within axle housing 108. As casing 16 rotates, the sides of holes 36, 38 bear against the portions of the cylindrical surface of cross pin 32 in the holes. The rotation of cross pin 32 about axis 48 causes pinion gears 28, 30 to revolve about axis 48. The revolution of the pinion gears about axis 48 causes at least one of side gears 24, 26 to rotate about axis 48, thus causing at least one of axles 40, 42 to rotate about axis 48. Engagement of the clutches as described above arrests relative rotation between the side gears and the differential casing.
Differential casing 16 is rotatably supported within axle housing 108 by means of identical first and second bearings 110, 112. Because of the proximity of ring gear flange 102 to the end of casing 16 nearest first bearing 110, in operation, that bearing is more heavily loaded than is second bearing 112.
Electromagnet 14 is rotatably supported on second differential casing portion 16b by third bearing 114. Electromagnet 14 is rotatably fixed relative to axle housing 108 and disposed in close proximity to casing 16, which rotates relative thereto. The voltage applied to electromagnet 14 to energize same and actuate primary clutch element 66 may be controlled by a control system (not shown) which is in communication with sensors (not shown) which indicate, for example, excessive relative rotation between axles 40, 42, and thus the need for traction control. Housing 108 includes hole 116 fitted with rubber grommet 118 through which extend leads 120. Through leads 120 the control system provides voltage to electromagnet 14. As electromagnet 14 is energized, a magnetic initiating force is applied to primary cone clutch element 66 by a toroidal electromagnetic flux path (not shown) which is established about the annular electromagnet coil 126; the flux path flows through ferrous casing portions 16a and 16b and through clutch element 66. Clutch element 66 is thus magnetically drawn into engagement with casing 16 during operation of electromagnet 14. Because it is made of a magnetic material (e.g., steel) and has a solid structure, primary cone clutch element 66 is better suited for conducting the magnetic flux path therethrough than would be a clutch comprising a series of interleaved discs, which may have gaps therebetween and which would likely be formed of materials which would not so readily transmit the magnetic flux. Further, casing part 16b may include annular nonmagnetic portion 122 to help direct the toroidal magnetic flux path through primary cone clutch element 66, as described in U.S. Pat. No. 6,019,694 (Forrest et al.), assigned to Auburn Gear, Inc., the disclosure of which is expressly incorporated herein by reference.
FIG. 2 depicts a second embodiment of a prior axle assembly which is identical in structure and operation to the above-described axle assembly 10 except as follows: Axle assembly 10xe2x80x2 comprises electromagnet 14xe2x80x2 which is fixed to the axle housing, rather than being rotatably supported by a bearing 114 disposed about casing part 16b. Bearing 110xe2x80x2 is disposed in cup 124 which extends inwardly of the axle housing to engage and support electromagnet 14xe2x80x2 in the manner described in pending U.S. patent application Ser. No. 09/484,967, filed Jan. 18, 2000. Notably, bearing 110xe2x80x2 is somewhat smaller than bearing 110 (and identical bearing 112) and, as noted above, would be more heavily loaded during operation than larger bearing 112 due to the proximity of the ring gear.
Although cone clutches of the type disclosed above are better suited than disc-type clutches as primary clutch elements in electromagnetically-actuated limited slip differentials, for the reasons set forth above, their load carrying capability is limited, for a give axial engagement force, by the angle of the included angle formed by the cone clutch engagement surfaces. Typically, these angles range from 9xc2x0 to 12.5xc2x0. The smaller this angle, the greater the torque capacity of the cone clutch. The smaller this angle, however, the harsher the clutch engagement, and the smaller the tendency for the clutch to release. Clutches having multiple interleaved discs, or xe2x80x9cclutch packs,xe2x80x9d are well known in the art and generally have greater torque capacity than a cone clutch of approximately equal package size. Moreover, the required tolerances associated with manufacturing cone clutches tend to be somewhat smaller than with disc clutches.
Further still, compared to the axial movement needed to engage disc clutches, a greater distance is needed when using cone clutches because a portion of the movement is absorbed by the casing as it is being radially stretched. Therefore, relatively more movement between the pinion and side gears is needed to accommodate proper movement of the cone clutch, and optimal gear mesh clearances therebetween, which are on the order of xc2x10.010 inch, may be compromised. An electromagnetically-actuated limited slip differential assembly which provides the respective benefits of cone clutches and clutch packs is highly desirable.
A further issue associated with electromagnetically-actuated limited slip differentials is that the electromagnet tends to magnetize ferrous components within the axle housing, particularly those in close proximity to the electromagnet. This can be of particular concern where relatively moving, interengaging components such as bearings or gears of the differential or axle assembly become magnetized and attract metal shavings or other ferrous debris, or where the shavings and debris are themselves magnetized and become attached to these interengaging components. The collection of such contamination on these components can substantially accelerate their wear and lead to premature failure.
One known approach to addressing this issue is to provide a magnetic drain plug in the axle housing, which may attract and retain some of the debris. However, the debris may be equally attracted to other magnetized components within the axle housing, rather than to only the drain plug. Another approach to addressing this issue is described in
U.S. Pat. No. 6,165,095, which discloses an apparatus and method for demagnetizing the components initially magnetized by the electromagnet. While effective, this means for demagnetization involves providing additional controls for directing current through the electromagnet(s). It is desirable to provide a simple and effective means for reducing the likelihood or severity of magnetization of at least some of the relatively moving, interengaging components within the axle housing.
Further, one way to reduce the cost and improve the reliability of an axle assembly is to reduce the number of components parts, or at least the number of complex, high precision parts. For example, reducing the number of ball or roller bearings may reduce the cost of material, the cost of assembly labor, and the number of moving parts, thereby improving durability and reliability. Reduction in the number of parts, however, may compromise the ability of the remaining parts to perform satisfactorily. For example, reducing the number of bearings may increase the load to be borne by the remaining bearings, which may adversely affect the durability of those remaining bearings. The reduction of costs without compromising performance is an ongoing and important goal in virtually every commercial endeavor, and means for accomplishing that goal are therefore highly desirable.
The present invention provides a differential assembly including a rotatable casing, first and second axially moveable side gears disposed within the casing, at least one pinion gear disposed within the casing and intermeshed with the side gears, a cone clutch operatively coupled to the first side gear, the cone clutch being frictionally coupled to the casing in response to being exposed to a magnetic field, and at least one clutch disc operatively coupled to the second side gear in response to axial movement of the second side gear.
The present invention also provides a differential assembly including a rotatable casing having opposite ends, a differential gear mechanism and a magnetically-activated clutch disposed within the casing, relative rotation of at least a portion of the gear mechanism being selectively frictionally engaged with the casing by the clutch, an electromagnet being disposed proximal to one of the casing ends, and a ring gear attached to the casing at a location proximal to the other of the casing ends.
The present invention also provides a differential assembly including a rotatable casing, a differential gear mechanism and a magnetically-activated clutch disposed within the casing, relative rotation of at least a portion of the gear mechanism being selectively frictionally engaged with the casing by the clutch, an electromagnet disposed proximal to the casing, the casing and the electromagnet having relative rotation therebetween, and a self lubricating bearing disposed between the electromagnet and the casing, the electromagnet being supported relative to the casing by the bearing.